Nicotine produces behavioral effects through a diverse family of nicotinic acetylcholine receptors (nAChRs). Nicotine-evoked dopamine release is thought to play an important role in the establishment and maintenance of nicotine dependence, which undelies peoples'continued use of tobacoo products, despite their well-known dangers to health. alpha-conotoxin MII is a toxin isolated from the predatory cone-snail Conus magus, which distinguishes between subsets of nicotinic receptors. In the first funding periods of this grant, we used alpha-CtxMII to identify and analyse the nAChR populations that modulate nicotine-evoke dopamine release. We also began to study the effects of chronic nicotine exposure on these receptors, and how they are affected in models of Parkinson's Disease. In order to enhance the usefulness of alpha-CtxMII, we have also engineered variants which have incorporate new properties or increased its selectivity for particular nAChR populations. Experiments outlined in the current proposal will extend these studies further. 1) Chronic treatment and nicotinic subunit mutation will be used (alone, and in combination) to investigate how the nAChR populations characterized in the previous funding period interact with each other, and with the neurotransmitter systems that they modulate. 2) New alpha-CtxMII derivatives will be developed with two aims. First, to make alpha3beta2-nAChR subtype selective derivatives (we do not have such a compound at present, but this is a natually-expressed nAChR subtype that requires further study). Second, to produce alpha-CtxMII-based radiolabels that retain the original selectivity of [125l]alpha-CtxMII (which was developed in the previous funding periods), but with improved assay performance. This will improve out ability to measure and study these important nAChRs. 3) Synthesize and characterize derivatives of alpha-conotoxin ArIB, which has selectivity for alpha7-subtype nAChRs. We intend to develop a series of useful, highly-selective tools for the study of this naturally-occurring nAChR subtype. The proposed studies will provide further insights into the locations, numbers and functional roles of naturally-occurring nAChRs, and the interactions between them. It is likely that this increased understanding will, in turn, illuminate which nAChR subtypes are implicated in nicotine dependence, and thus assist in attempts to develop more-effective smoking cessation aids. These insights may also guide the design of nicotinic therapies for other conditions.